The present invention concerns a flat resistance heating element.
Electrical resistance-heating elements find manifold uses, for instance for room heating. Relative to heating elements having resistors in the shape of rods, tubes or coils, those having flat resistive planes prove to be particularly advantageous, since they can give off heat all across the surface of a resistive layer.
In some areas, for instance in old buildings, it may be necessary to provide high-power flat resistance heating elements. However, at the same time such a flat resistance heating element must also be able to serve without safety risks, even when mechanically damaged or exposed to environments with water splashes.
It is the task of the present invention to provide a flat resistance heating element, subsequently simply designated as heating element, which satisfies these requirements, can be operated with line voltage, can moreover be installed and electrically connected in a simple way, and in which several electroconductive layers are provided which have contact electrodes inserted or applied in such a way that only a selected number of contact electrodes is reached when the heating element is provided with contacts at a given position.
From the German Offenlegungsschrift (open patent application) OS 2449676, a grounded flat resistance heater is known that can be protected through an earth-leak circuit breaker (ELCB). It consists of an insulating supporting sheet having on one side a conductive coating that serves as heating layer and is to be connected to the electrical power supply, and on the other side a further electroconductive coating serving as a layer to be grounded. One can choose to also coat the peripheral conductive layers with electrically insulating layers.
It is an incisive disadvantage of this kind of flat heating element that in the operating state, i.e., with the power connected, a capacitive feedback always develops between the heating layer and the grounded layer which, depending on the size of the heating layer, gives rise to a more or less pronounced leakage current at the grounded layer. This means that depending on the size of a given heating element, a suitable earth leak circuit breaker or ELCB must be selected or, conversely, the size of a desired heating element must be adapted to the leakage-current tolerance of a given ELCB in order to prevent premature and undesired triggering of the ELCB.
A similar kind of flat heating element is described in the German Auslegeschrift (open patent application) AS 1288702. There, too, a resistive heating layer is separated from a second conductive layer, particularly a protective metal foil that is to be grounded, by an insulating layer, and where the layer to be grounded either is itself realized as a fusible cut-out consisting of readily fused metal, or where a fusible cut-out is inserted into the circuit of the heating layer and/or the grounded layer. In this type of flat heating element, too, the peripheral conductive layers can be covered by insulating layers. As a further protection, an additional metal foil formed as fusible cut-out and also separated from the heating layer by an insulating layer can be applied to the backside of the heating layer.
This type of embodiment of a flat heating element, which originated in the year 1957, aims particularly at measures providing the best possible assurance of electrical safety of the installation for the user at that time, and which were supposed to cause a sufficiently rapid interruption of the circuit in the case of possible short circuits. Nowadays this safety engineering problem is tackled much more elegantly and reliability by the earth leak circuit breakers (ELCB) that have since been developed.
The present invention, to the contrary, is based on recognizing that complete independence between the make of the ELCB and the size of the heating element can be achieved when the ground lead (safety ground) is kept essentially completely free of capacitive leakage currents. Relative to known flat heating elements, this has rather substantial advantages, both for practical applications and for the authorization procedures or certification procedures defining the level (class) of protection according to standard specifications of the competent technical inspection bodies. With it, it is for the first time possible to fit buildings or other objects with flat electrical resistance heating units of arbitrary size without the need to pay special attention to the ELCB, which in most cases has already been installed. Also, in contrast to existing types of flat heating elements, a certification of the heating elements according to the invention with respect to their level (class) of protection can be performed and issued regardless of size. Moreover, the capacitve leakage current in the ground conductor (safety ground) would also constitute undesirable interfering parameters because of the additional phase shifts in the ELCB region.
The cited disadvantages of the state of the art can be overcome and the precited advantages of the present invention can be attained according to the invention by a heating element according to claim 1. In the case of mechanical damage to the heating element, for instance, a circuit breaker or earth-leak circuit breaker can be triggered by the safety ground. Through this first conductive layer which functions as an additional neutral conductor, a capacitive coupling between the resistive heating layer and the safety ground is fundamentally suppressed. The neutral conductor screens the safety ground capacitively with respect to the resistance heating layer. The flat neutral conductor and the superimposed, flat safety ground are essentially at the same potential. Hence, a big capacitive leak current cannot flow via the safety ground between these two flat conductors, regardless also of the size of the total resistance heating element, which might be composed of individual elements.
Further embodiments are outlined in claims 2 to 16.
For the heating element to be put into operation, one contact electrode of the resistive layer is connected to the neutral conductor, the other is connected to phase, so that a current flow is generated in the plane of the resistive layer causing it to warm up and give off the heat to the surroundings.
Because of its design, the heating element according to the invention can be contacted with simple means. Thus, electrical contact can be made to the heating element according to the invention by inserting contact elements, for instance contact tips, extending through the thickness of the heating element. Such a contact tip is electrically connected, either to phase or to the neutral conductor or to the ground of the power supply; when such a contact tip is inserted into the heating element according to the invention, it is exclusively connected with the desired contact electrodes of a given layer. A short circuit between the individual contact electrodes can thus be avoided.
Beyond that, the design of the heating element according to the invention also permits a gradual or positive engagement between the power supply cable and the contact electrodes to be established. Such a connection can be brought about by contacting means making in-depth contact with the contact electrodes. In this case clamps can be used which engage from above and below via electroconductive contact blades or contact teeth introduced at predefined locations into the heating element. This in-depth contact is only possible with the heating element according to the invention. If an additional safety ground or a screening was applied to a traditional heating element, a short circuit would be brought about between the individual layers through the pressure applied to introduce the contact element and through the contact element itself. Apart from the precise connection of predefined contact electrodes, this in-depth contact has the additional advantage that the positive engagement between heating element and power supply cable can also support tensile stress and shear stress.
The heating element according to the invention can be powered with line voltage, hence the installation requirements for such a heating element are low. Transformers and other big components that would be needed for low-voltage elements can be dispensed with when using the heating element according to the invention. In view of these low installation requirements, a multitude of applications open up for the heating element according to the invention.
A direct contact between contact electrodes is avoided over the entire length of the heating element by the arrangement of the contact electrodes envisaged according to claim 3.
According to the embodiment of claim 4, the entire heating element can be enclosed and sealed against humidity by insulating layers arranged peripherally, and risks arising when touching the surface heating element can thus be avoided.
According to the embodiment of claim 4, the entire heating element can be enclosed and sealed against humidity by insulating layers arranged peripherally, and risks arising when touching the flat resistance heating element can thus be avoided.
Preferred materials for the resistive mass are described in claim 5. It is one advantage amongst others when using an appropriately selected, electroconductive polymer in the resistive mass that the power of the heating element can be raised relative to that obtained when using carbon black.
The embodiment according to claim 7 has the further advantage, apart from advantages with respect to production engineering, that the entire heating element has a high flexibility, for instance when electroconductive polymers are employed, and that on account of the elasticity, it is stable against mechanical loads and thermal fluctuations and that it is readily stored, transported, and installed without mechanical damage.
According to the embodiment following claim 9, the heating element has openings which can for instance be circular in shape, and make it possible to fix the heating element at the wall or floor for instance. A fastening part, for instance a screw, can be passed through the openings without causing a short circuit of the conductive layers and the resistive layer.
The design of the heating element according to claim 10 offers possibilities of contact at different points of the heating element. For instance, depending on the dimensions of the zone to be covered by the heating element, the appropriate contact electrodes from which the path to the current supply leads is shortest can be selected in each layer.
In this connection the embodiment according to claim 11 is preferred, because in this way one can achieve heating over the entire surface area of the subdivisions that is delimited by the contact electrodes.
In an embodiment according to claim 12, the zone to be heated can be adjusted to the width of the heating element, while in an embodiment having several pairs of contact electrodes in the resistive layer, this width can be varied between the spacing of a contact electrode pair and the total width of the heating element.
According to a particularly appropriate embodiment according to claim 13, cutting lines along which the heating element can be subdivided are created by band-shaped gaps between the subdivisions of each layer. When a heating element is cut apart in such a zone that is free of resistive mass or conductive material, renewed possibilities for making contact arise because of the continuous contact electrodes. The heating element according to the invention can thus be cut down to any size needed without losing the advantages of the contact electrodes protruding beyond the subdivisions and the attendant possibilities for making contact.
Preferably, in this case the subdivisions are arranged according to claim 14, so that it is guaranteed when cutting up a heating element according to the invention that along the cut, none of the subdivisions in the resistive layer or in the first or second conducting layer are openly exposed, that is, not insulated; therefore, contacts can be provided without any risk.